1. Technical Field
Embodiments of the present disclosure are related to the formation of ball grid arrays in and on semiconductor packages, and in particular, to methods and mechanisms for positioning solder balls, especially in relation to very fine pitch arrays.
2. Description of the Related Art
Ball grid arrays are used in many kinds of semiconductor device packages, both internally and externally. Many different systems and processes have been developed for positioning the solder balls on the devices, including pick-and-place systems, vacuum devices, stencils, solder paste printing, and plating operations. As technologies have improved, and the pad pitch of arrays has reduced, it has become more and more challenging to accurately and reliably position solder balls on contact pads to form a ball grid array.
FIG. 1 is taken from U.S. Pat. No. 6,253,992 to Fjelstad, issued Jul. 1, 2001, and shows one known system for placing solder balls. The system depicted includes a stencil 10 having a plurality of holes arranged to correspond to the arrangement of contacts on a chip on which solder balls are to be positioned. An escapement element 26 that includes a reservoir defined by side walls 24 rests on the stencil 10. The escapement element 26 also has a plurality of holes that are arranged to correspond to the holes of the stencil 10, so that when the reservoir and the stencil are properly positioned relative to each other, each hole of the stencil aligns with a corresponding one of the holes of the escapement element, so that a solder ball 16 can drop from the reservoir through the holes of both the escapement element and the stencil. An actuator mechanism 40 is coupled to both the escapement element 26 and the stencil 10, and is configured to move the escapement element relative to the stencil, so as to bring the holes of the respective elements into and out of alignment. Standoffs 20 on the bottom of the stencil 10 maintain a selected spacing between the stencil and an underlying surface. The stencil 10 is linked to a chip support 42 to enable proper positioning of the stencil relative to the contact pads of a chip.
In operation, the actuator 40 holds the escapement element 26 out of alignment with the stencil 10 while a semiconductor chip 50 is positioned on the support 42. The escapement/stencil assembly is then lowered onto the front surface 54 of the chip 50 with the holes of the stencil 10 positioned above the contacts 52 of the chip. The actuator 40 moves the escapement element 26 into alignment with the stencil 10 so that a solder ball 16 drops through each of the holes onto the corresponding contact 52, where it is held in place by a thin layer of flux previously deposited over each contact. The standoffs 20 hold the stencil 10 at a height that permits only one solder ball 16 to drop from each hole. The actuator 40 then moves the escapement element 26 out of alignment, closing the holes, and the assembly is lifted from the chip 50 and the process repeats.
The process described above is one of many that are employed to emplace solder balls. Stencils similar to the stencil 10 of FIG. 1 are also used in various other processes for ball placement.